1. Field of the Invention
This invention relates to a method and apparatus for the controlled aeration of stored grain. More particularly, the invention relates to a method and apparatus for sensing ambient temperature and relative humidity conditions and selectively aerating grain when suitable or best-available ambient temperature and relative humidity conditions are present.
2. Description of Prior Art
Mold is the major cause of spoilage in stored grain. Mold growth occurs when a moisture and temperature environment suitable for mold is present around the stored kernels. Foreign matter, along with higher temperatures and higher humidities, provide the most favorable environment for mold growth. Clean grain can be stored indefinitely in a storage bin if its moisture and temperature are kept within acceptable limits.
Moisture can be introduced into the air spaces around stored grain by condensation or by the natural respiration of the grain. Condensation can occur when relatively warm, moist air is introduced into the bin and comes into contact with grain that is colder than the air. Condensation more frequently occurs as a result of moisture migration, which happens when natural convection currents within the bin bring warm air from one region of the bin into contact with cooler grain in another region. Crusting and spoiling can result. It is known that the effects of condensation can be minimized by keeping the temperature of the grain at or near the average ambient air temperature.
Moisture introduced into stored grain from the natural respiration of the grain is a function of the temperature and relative humidity of the air surrounding the grain. For a specified temperature and relative humidity combination of the surrounding air, there is a corresponding equilibrium moisture content for the grain; that is, if the air surrounding the grain is kept constant at the specified conditions, the grain will eventually reach the equilibrium moisture content. Moisture will be given off by the grain kernels when the moisture content of the grain exceeds the equilibrium moisture content supported by the surrounding air conditions; conversely, moisture content will increase when surrounding air conditions will lead to an equilibrium moisture content higher than that present in the grain kernels. In this regard, it should be noted that mold attacks a grain kernel from the outside in; it is the presence of excessive moisture on the outside of the kernel that is to be avoided.
Mold growth on stored grain, then, can be restricted by controlling the moisture content and temperature of the grain. The grain temperature and moisture content determine the allowable storage time that the grain can be kept before it spoils. For that reason (and others), grain prices are adjusted for the moisture content of the grain. Grain which has an excessive moisture content must either be dried, or used quickly, and is therefore of less value than grain marketed at standard moisture content levels.
The effects of condensation can be controlled by maintaining the stored grain at a temperature equal to the temperature of the surrounding air. The effects of moisture release due to respiration could be avoided by excessively drying the grain. Excessive drying of grain, however, is undesirable for several reasons. First, grain that is at or below its equilibrium moisture content for the ambient air conditions, will not spontaneously give off moisture. It requires energy to remove each additional increment of moisture from a kernel as the kernel dries, and overdrying of the grain below its desired market moisture content consumes energy at an increasingly faster rate as the drying progresses. Secondly, overdrying of grain creates internal stresses within the individual grain kernels, causing cracks and fines, thus lowering the quality of the grain and its market value. Finally, grain is marketed by weight. Overdrying of grain removes more water than is necessary, thereby reducing its total weight. To maximize price, as much moisture should be retained in the kernels as possible, keeping in mind the upper allowable moisture content for safe storage and marketing standards.
Proper storage of grain, then, involves two primary considerations. First, the temperature of the grain should be as close as possible to the temperature of the air surrounding the grain, to avoid moisture migration and condensation. Secondly, the moisture content of the grain should be kept at a predetermined moisture content level that maximizes the weight of the grain at market time, yet is low enough to restrict respiration. A secondary, economic consideration is that aeration used to maintain temperature and humidity should not be performed more than necessary, as extensive aeration fan operation can lead to high energy costs.
U.S. Pat. No. 3,563,460 to Nine discloses a means for controlling the aeration of stored grain. The Nine device incorporates a plurality of temperature sensors located within the grain, and a comparison device for comparing the monitored temperature to a manually set temperature level. An aeration fan is activated when the grain temperature exceeds the set level. The Nine device, however, requires a continual manual adjustment of the set temperature level in order to maintain the grain temperature reasonably near the actual or average ambient temperature. Moreover, the Nine device does not include any mechanism, manual or automatic, to control aeration of the grain as a function of the relative humidity of the ambient air.
U.S. Pat. No. 4,045,878 to Steffen discloses a method for aerating stored grain wherein the stored grain is exposed to a throughput of atmospheric air if the current atmospheric conditions are optimal, in that they are at or near predetermined historical monthly average atmospheric conditions. Although the method disclosed in the Steffen patent, at least in theory, takes into consideration both temperature and relative humidity, application of the method has several drawbacks. First of all, continuous operator monitoring of ambient air conditions is required. Secondly, aeration of grain is premised on historical monthly temperature averages, and not on the actual current average temperature, which can vary considerably from historical seasonal averages. Finally, long periods of time may elapse without any aeration of the grain at all if the predetermined optimal air conditions are not met.
A method for aeration of stored grain that is responsive to the actual (as opposed to historical) average temperature, which provides for controlled aeration of the stored grain even when long periods of less than optimum conditions have elapsed, and that can be implemented by an automated apparatus, would be a decided advantage.